Serotonin (5-hydroxytryptamine; 5-HT) is a prominent neurotransmitter that modulates a wide variety of sensory, motor, and behavioral responses in the mammalian nervous system. Serotonergic synapses are major targets for the action of psychotropic and antidepressant drugs like LSD, psilocybin, and Prozac and it is therefore believed that transmission at these synapses plays an important role in the regulation of mood, behavior, and perceptual states. Indeed, drugs that modify transmission at synapses are presently used in the management of depression, obsessive-compulsive behavior, eating disorder, anxiety, migraine headache, and chemotherapy-induced nausea. In addition to its well established role as a mediator of synaptic transmission in the adult nervous system, a number of independent observations suggest that serotonin may also function during embryogenesis to modulate development and morphogenesis in neural and non-neural tissues. The diverse responses to serotonin are mediated through its interaction with at least fourteen distinct and surface receptor subtypes. The complexity of this signaling system and the paucity of selective drugs have made it difficult to define specific functions for individual 5-HT receptor subtypes, or to determine how serotonergic drugs modulate mood and behavior. The long-term goal is to address these issues by using genetic methods to define roles for serotonin receptor subtypes in the regulation of developmental, physiological, or behavioral states. The primary focus of this research proposal is to elucidate functional roles for the 5-HT-3 receptor, a serotonin-gated ion channel that mediates rapid excitatory responses in central and peripheral neurons. The first specific aim is to generate transgenic mice that lack functional 5-HT3 receptors. This will be achieved by targeted disruption of the endogenous 5-HT3 receptor gene through homologous recombination, or by ectopic expression of a truncated amino-terminal fragment of the 5-HT3 receptor subunit that can inactivate native ion channel complexes via a dominant-negative effect. 5-HT3 receptor deficient mice will be examined for developmental, physiological, or behavioral abnormalities compared to wildtype siblings. The second specific aim is to identify cis-acting DNA elements and trans-acting nuclear factors that regulate 5-HT3 receptor gene transcription in neurons or neuroendocrine cells. This will be accomplished by using gene transfer and transgenic methods to pinpoint promoter elements upstream of the 5-HT3 receptor transcript that confer regulated expression of this gene in vitro and in vivo. Candidate transcription factors will be examined for their potential to interact with these promoter elements. The goal is to use the 5-HT3 receptor as a model system for studying mechanisms that regulate gene expression in the nervous system, and to identify specific promoter elements that may be used to direct the ectopic expression of heterologous proteins in the nervous system.